Nadal improved to 16-0 this year on the hard surface Monday, defeating American Ryan Harrison 6-4, 6-2, 6-2 in the first round of the U.S. Open.

"It's difficult to analyze now," Nadal said, when asked if he's playing the best hard-court tennis of his career. "I am playing well. Happy the way I was playing. I don't know if it was the best or not, but it was very good because I was able to win."

The victory in front of a half-full Arthur Ashe Stadium marked the first for second-seeded Nadal on any Grand Slam surface other than clay since the first round of Wimbledon in 2012.

Since then, Nadal has fallen to Lukas Rosol at the All England Club, withdrawn from the 2012 U.S. Open (knee) and 2013 Australian Open (stomach ailment), won the French Open and fallen to 135th-ranked Steve Darcis in the first round of this year's Wimbledon.

Nadal answered that unexpected loss with 11 straight wins, all on hard court, to improve to 16-0 on the surface this season. He won titles at Montreal and Cincinnati in the lead-in to the U.S. Open, which he won for the only time in 2010.

"We go day to day," Nadal said. "Two tournaments before winning is great for my career because both tournaments are very important ones. But that (doesn't) mean I will play great here. That means that I am doing the right things."

Harrison won't argue that point.

The 21-year-old American got bullied around the court by Nadal, who hit 28 winners and forced Harrison into 32 errors.

"From what I see, and what I gather, these top guys are really, really good at peaking at the slams," Harrison said. "They know how to get their best out of it."

The Nadal match was never close, but Harrison said he isn't losing hope.

"If you're 30 years old and you've only got a number of Slams left, then maybe you're upset with it," Harrison told The Associated Press on Sunday, when asked about his draw. "I'm 21. I'm excited. Every single time I get to play a match like this, it's hard not to be excited. ... And if I don't want to play the top guys in the first round, then that's kind of an incentive to get seeded."

Earlier Monday, British qualifier Daniel Evans stunned 11th-seeded Kei Nishikori in straight sets for the first big upset of the tournament.

The 179th-ranked Evans won 6-4, 6-4, 6-2. He was making his debut at Flushing Meadows and had been 0-2 in Grand Slam matches, both at Wimbledon. The 23-year-old had to rally from a set down in his final match of qualifying to make the field.

"I was pretty calm today," Evans said. "It wasn't that much of a big deal what was happening on the court. I wasn't nervous serving it out."

Also 23, Nishikori was already playing in his 17th major tournament. He was a quarterfinalist at the 2012 Australian Open.

Both players faced nine break points Monday, but Evans converted six to just two for Nishikori.